Sterilization processes are utilized to sterilize a wide variety of materials including, for example, medical instruments, surgical instruments, and the like. Items to be sterilized are typically placed in a chamber and subjected to conditions believed to be sufficient to effectively sterilize the items and render them free (or at least to a pre-determined, acceptable level) of biological contaminants. There are a variety of sterilization techniques by which sterilization can be effected including steam sterilization, exposure to gaseous sterilants (e.g., ethylene oxide, vaporized hydrogen peroxide, and the like), plasma sterilization, and the like. Regardless of the techniques utilized to sterilize items, evaluating the effectiveness of the applied sterilization process is beneficial to ensure that the process provided the desired degree of sterilization. Evaluating the effectiveness of a process may be particularly desirable when sterilizing items such as medical instruments and devices invasive to the human body.
The efficacy of sterilization processes are evaluated using sterilization indicators, which typically evaluate whether a sterilization resistant challenge material survives a sterilization process. A typical biological indicator system, for example, includes a source of microorganisms (e.g., bacterial spores), a culture medium, and a visible detector to indicate the presence or absence of viable microorganisms. The indicator system is subjected to a sterilization cycle, which should be sufficient to kill the microorganisms. Following the sterilization cycle, the source of microorganisms is combined with the culture medium and then incubated to encourage the outgrowth of any remaining viable microorganisms. During the incubation period, the indicator system is evaluated to determine whether any microorganisms survived the sterilization process. The indicator may be evaluated visually (e.g., by turbidity or a color change) or with a detector (e.g., by spectroscopy using a spectrophotometer, fluorometer, or the like), to measure a selected property such as pH change, fluorescence, change in light absorbance, and the like.
Commercially used biological indicators frequently employ a system in which the culture medium is separated from microorganisms by placing the growth medium in a glass ampoule and disposing the ampoule within a container housing the microorganisms. Following the sterilization process, the biological indicator is activated by breaking the ampoule, which releases the growth medium into the container.
Commercially used biological indicators may also have relatively long incubation periods to obtain a detectable level of spore outgrowth. For example, commercially used biological indicators may require incubation periods from eighteen hours to up to seven days. Depending on the items being sterilized, such long periods for evaluating the efficacy of a sterilization process may not always be practical. In particular, medical devices and instruments that have been sterilized should not be used while still evaluating the efficacy of the sterilization process to which the devices were subjected. But it is costly to have medical devices inactive for extended periods while determining if they have been sufficiently sterilized.
To provide a more rapid indicator to evaluate sterilization efficacy, some systems evaluate the activity of enzymes occurring in microorganisms rather than microorganism growth. For example, 3M Corporation makes a rapid readout indicator under the tradename ATTEST®, which utilizes an enzyme that occurs naturally in the spore coat to degrade 4-methylumbelliferyl-α-D-glucoside to a fluorescent breakdown product. The fluorescence signal associated with this enzyme can be measured within one to three hours. In this indicator, a non-fluorescent substrate is added to the media, the substrate degrades to produce a fluorescent compound, and the fluorescent compound, rather than microorganism outgrowth, is monitored to evaluate the process. These indicators are utilized for evaluating steam sterilization processes. During sterilization, the steam heat inactivates the enzyme that performs the non-fluorescent to fluorescent reaction.
Other examples of biological indicators that employ enzymes whose activity is correlated with spore viability to give an indication of sterilization efficacy include those described in U.S. Pat. Nos. 5,073,488; 5,223,401; 5,418,167; 5,866,356, and 6,566,090.